1. Field of the Invention
The present invention relates to a wireless receiver apparatus comprising an automatic gain control function.
2. Description of the Related Art
FIG. 1 exemplifies a frame format used for a wireless telecommunication transmission system.
Referring to FIG. 1, one frame is comprised by one reference symbol and five data symbols, with the reference symbol being time-multiplexed in relation to the data symbols at a constant interval. Note that there is no need of five data symbols existing relating to one reference symbol, and other comprisals are also the same. The term “reference symbol” corresponds to “pilot symbol” in the W-CDMA system, whereas the present specification uses the term “reference symbol” in order to discuss more generally.
A wireless receiver apparatus performs a channel estimation and demodulates a data symbol by compensating for a channel response. A channel estimation circuit calculates a correlation between reference symbols received at a constant interval and a known reference pattern, thereby performing a channel estimation. A channel compensation circuit compensates for a channel response of data symbols by using a channel estimation value.
Here, in the case of demodulating data symbols of a certain frame, a channel estimation value acquired from the reference symbol of the same frame is utilized. Or a use of the reference symbol of an adjacent frame in addition to the reference symbol of the same frame enables an improvement of channel estimation accuracy. For instance, it is possible to suppress a noise component by averaging channel estimation values acquired from the respective reference symbols of two frames and thereby improve channel estimation accuracy. Alternatively, an interpolation of two channel estimation values makes it possible to estimate a channel estimation value at each data symbol position highly accurately even in the case that a variation amount of a channel response changes faster as compared to a frame length due to a Doppler effect.
FIGS. 2 through 4 are block diagrams exemplifying a configuration of a wireless receiver apparatus used for a wireless telecommunication system.
A common wireless receiver apparatus is equipped with an automatic gain control function (AGC) for varying a gain according to a level (i.e., a power or a voltage) of a received signal, and comprises an antenna 10, an AGC amplification unit 11, an AGC control unit 12, an A/D (analog-digital) converter 13, a channel estimation unit 14 and a channel compensation unit 15 as shown in FIG. 2. Note that the AGC control unit 12, channel estimation unit 14 and channel compensation unit 15 are included in a signal process unit 16 and are disposed for digitally processing a digital signal by using a DSP, et cetera.
The AGC control unit 12 raises a gain of the AGC amplification unit 11 (i.e., an AGC gain) when an input level is low relative to a certain target level, while it lowers the AGC gain when the input level is high relative to the target level, thereby controlling a received signal level at constant prior to the A/D converter 13 and accomplishing a wide dynamic range.
As for the AGC function in this part, the control is carried out by digital processing, in lieu of controlling by way of an analog AGC voltage, in order to improve accuracy of a gain control. This configuration makes an AGC gain change in steps relative to a received level.
FIG. 3 is a block diagram showing an internal configuration of the AGC control unit 12.
An input level measurement unit 17 comprised by the AGC control unit 12 measures an input level of a digitalized received signal sent from the A/D converter 13 and inputs input level information to an AGC control value calculation unit 18 in which a target level is provided as a parameter value and an AGC control value is calculated here so as to make the input level close to the target level. The calculated AGC control value is sent to the AGC amplification unit 11 and used for controlling an AGC gain thereof.
FIG. 4 is a block diagram showing an internal configuration of the channel estimation unit 14.
A digitalized received signal from the A/D converter 13 is input to a channel estimation value calculation unit 20 which then calculates a channel estimation value by using a reference symbol. The calculated channel estimation value is input to a compensation coefficient generation unit 21 which then calculates a channel compensation coefficient to be provided to the channel compensation unit 15. A method for calculating a channel compensation coefficient includes the one as described above.
A receiver apparatus using a conventional channel estimation includes the one as noted by a patent document 1 which notes a technique for retaining a gain of an amplifier for each period between a reference symbol to the next one and making an updating timing of the gain at a positive multiple timing of a reference symbol.
[Patent document 1] Japanese Patent Application Publication No. 2003-92561
An AGC function of a common wireless receiver apparatus changes an AGC gain at a timing a synchronously with a main signal. And it demodulates a data symbol without considering a fact of an AGC gain being changed also when performing channel compensation. The above described results in the problem of an error rate characteristic being deteriorated if AGC gains are different between a data symbol to be demodulated and a reference symbol for calculating a channel estimation value.
And if AGC gains are different among a plurality of reference symbols for performing channel estimations, a channel estimation value is added by an amplitude fluctuation effect due to the difference of the AGC gains, thereby resulting in the problem of degrading an error rate characteristic if a channel compensation is carried out by averaging or linearly interpolating the channel estimation value as is.